In the field of ophthalmology, a so-called photosensitizer and electromagnetic radiation can be used to alter the biomechanical and biochemical properties of eye tissue, namely the cornea, for example, for therapeutic purposes.
The human eye is delimited by the outer coat of the eyeball. The intra-ocular pressure tensions the outer coat of the eyeball, which contains collagen, and gives the healthy eye its approximately spherical shape. In the rear region of the eye, the outer coat of the eyeball is formed by the white sclerotic coat (sclera). The cornea, which is permeable to visible light, is located in the anterior region. A deformation of the outer coat of the eyeball can be the cause of defective vision. For example, one form of short-sightedness, axial myopia, can result from a sclerotic axial elongation of the eye. An ellipsoidal surface of the cornea can lead to a form of astigmatism, which is referred to as keratoectasia. Keratoconus is a further disease of the cornea. Keratoconus involves an unnatural degeneration of the cornea, which can lead to a progressive thinning and conical deformation of the ocular cornea. As the convexity increases, the cornea becomes thinner, e.g., underneath the center, depending on the progression of the keratoconus, whereby the cornea can perforate and scar over. Visual acuity is permanently diminished as a result.
In order to treat an advanced case of keratokonus, the diseased cornea can be removed using the keratoplasty method and can be replaced with an allograft. Such an operation is an organ transplant, however, and has associated risks and complications. After a case of keratoconus has been treated, it may take years after the keratoplastic operation for eyesight to return to an acceptable level.
According to a different type of therapy, after the early detection of keratoconus, the keratoconus is treated by stabilizing the cornea by cross-linking a photosensitizer that has been applied onto and/or into the eye. The aforementioned treatment results in a photochemical, non-tissue abrading stabilization or alteration of the biomechanical and biochemical properties of the cornea.
A photosensitizer or a photosensitizer solution is first applied, e.g., as an active agent, into the eye tissue to be altered and is then exposed to the radiation of a light source. Electromagnetic radiation in the wavelength range from approximately 300 nm to 800 nm (UV-A radiation or visible light) can be used, for example, as the luminous radiation or primary radiation.
Possible photosensitizers that can be used are, for example, riboflavin (vitamin B2), lysyl oxidase, transglutaminase, sugar aldehydes, ethylcarbodiimide, glutaraldehyde, formaldehyde, or mixtures thereof, e.g., Karnovsky's solution. The photosensitizer can be in the form of a liquid and/or gaseous solution or a powder. In conventional techniques, the epithelium of the cornea can be at least partially removed, for example, by means of an alcohol-containing agent, or the flap (epithelium of the cornea with stromal tissue) can be folded open by means of a so-called flap cut, in order to enable a photosensitizer to freely penetrate the cornea, since, depending on the photosensitizer solution that is used, the epithelium of the cornea can be a barrier for the diffusion of the photosensitizer molecules into the corneal tissue. The removal of the epithelium, for example, using the alcohol-containing agent, is usually painful for the patient and the subsequent healing process is not always without complications.
According to a recent approach, in order to accelerate the healing process, at least one channel is cut in the eye tissue with the aid of laser radiation before the photosensitizer is applied, said channel being cut in the cornea or the stroma thereof, for example, and extending from the surface of the eye tissue into the interior thereof, for example. Next, the photosensitizer can be applied in drops onto the eye and can thereby be introduced through the channel that was cut out and can diffuse into the eye tissue. In an alternative embodiment, the photosensitizer can also be introduced into the eye tissue in a targeted manner by means of a cannula. Forming the flap or the at least one channel by means of an incision is still a surgical intervention in the eye tissue. The disadvantage thereof is that forming channels by cutting only enables channel-type regions of the eye tissue to be supplied with the active agent, since certain stabilization areas must remain between the channels in order prevent further weakening of the eye tissue. Therefore, the only way to apply the photosensitizer over the entire surface is to remove the epithelium by means of the alcohol-containing agent or by means of a flap cut, as described above. In addition, when drops are applied, the photosensitizer is usually dosed manually, which can result in dosage fluctuations that cannot be reproduced.